The temperature difference between March and May predicted leaf-out intervals for four tree species at 111 Swiss stations in models with station and time effects, 1960–2025

We tested whether warm springs reduce the interval between leaf-out dates for hazel, beech, larch and spruce. Open MeteoSwiss files gave 4,664 complete station-years at 111 Swiss stations in 1960–2025. The original model used station effects, a time trend and March–April warmth. Its interval slope was +1.16 days per °C (95% year-block interval +0.08 to +2.61), but it was uncertain without the trend and in 2000–2025. A planned model with March–April and April–May warmth gave opposite slopes. In a secondary analysis, we expressed this model as common warmth and the March-minus-May temperature anomaly. The difference coefficient was +1.66 days per °C (+1.33 to +1.97), with estimates of +1.60 in both 1960–1999 and 2000–2025. A common 1 °C increase across the three months had an uncertain coefficient of −0.19 days (−0.89 to +0.63). Predictions for held-out decades had an error of 9.93 days with the temperature difference, compared with 10.44 days for station effects and trend alone. The same four species occur in every included station-year. Year-block resampling keeps common weather shocks together. Source data, code, plans, all results and figures are included. The secondary model choice used the full record. These are observational and retrospective results for the sampled network. They do not prove a biological mechanism or forecast the effect of future uniform warming.

phenology · leaf-out · Switzerland · MeteoSwiss · seasonal temperature · species timing · reproducible analysis